A key objective in well cementing is to isolate the different formation layers traversed by the well to prevent migration between the different geological layers or between the layers and the surface. In particular, it is essential from the point of view of safety to prevent any gas from rising through the annulus between the well wall and the casing.
When the cement has set, it is impermeable to gas. Because of the hydraulic pressure of the height of the cement column, the injected slurry is also perfectly capable of preventing such migration. However, between the two states, there is a critical phase that lasts up to several hours during which the cement slurry no longer behaves as a liquid but also does not yet behave as an impermeable solid. For this reason, the industry has developed a series of additives aiming at maintaining a gas tight seal during the whole cement-setting period.
Among such (numerous) additives are those which also tend to reduce fluid loss, i.e., prevent dehydration of the petroleum industry fluid when the latter comes into contact with a natural porous or fissured formation. This loss of water can impair the proper placement of slurry in the annulus due to drastic increase in its rheological parameters; plastic viscosity and yield stress. In general, cement slurries with a fluid loss of less than 50 ml over thirty minutes, measured in accordance with API (American Petroleum Institute) standards, are also impermeable to gas, although the correlation between the two phenomena is not systematic, and other parameters such as the almost complete absence formation of free fluid (although known as free water) is also necessary, especially in a non vertical slanted well since the supernatant free fluid can create a path for the migration of gas.
The fluid loss is controlled by adding to the cement slurry either high molecular-weight water-soluble polymers or particulate additives such as latices or crosslinked polymers.
The efficiency of water-soluble polymers is generally limited because they cannot be used at high concentrations owing to too high slurry viscosities at the mixing stage. This may be a major limitation when the solid volume fraction of slurry is high (e.g. higher than 50%) and/or when the slurry is designed for elevated temperatures since fluid loss control provided by these polymers is primarily based on a thickening effect of the interstitial water of slurry.
Some latices provide excellent fluid-loss control (API value below 50 mL/30 min.) and, therefore, are frequently used for gas migration control. However rather high concentrations are required, making latices not cost effective when there is no risk for gas migration. It is believed that small latex particles (around 150 nm diameter) fill the pores of cement filter cake, and can coalesce to form an impermeable layer of polymer. It may be difficult to increase the plastic viscosity of latex cement slurries that generally are thin. In some cases it can be difficult to properly remove the drilling mud when the slurry viscosity is low.
Though the use of latices, such as natural rubber latex, in Portland cements was common since the 1920s, especially because of the improvements in mechanical performance, a key improvement occurred in 1985, when Parcevaux et al. identified styrene-butadiene latex an effective additive for preventing annular gas migration. This system is known for instance from European Patent 0 091 377 (or its counterpart U.S. Pat. No. 4,537,918) that more specifically discloses cement slurry compositions inhibiting pressure gas channeling in the cemented annulus, even at high temperature, consisting essentially of a hydraulic cement, about 5-30% by weight of cement of a compatible styrene (70-30 weight percent)/butadiene (30-70 weight percent) copolymer latex and about 1-20% by weight of latex of a latex stabilizer and water in an amount such that the total fluid content of water, latex and stabilizer is about 30-70% by weight of cement.
In an attempt to control gas channeling at lower cost, U.S. Pat. No. 5,099,922 (Ganguli) describes the use of a copolymer of 5 to 95 weight percent 2-acrylamido-2-methylpropane-3-sulphonic acid (AMPS); (2) 5 to 95 weight percent of vinylacrylamide; and (3) 0 to 80 weight percent of acrylamide. This copolymer may be used in combination with a gas channeling inhibiting amount of an experimental styrene/butadiene latex, wherein the styrene is substituted with at least one selected from the group consisting of —COOR, —SO3R, and —OH, wherein R is H or a C1-C5 alkyl groups. However, there is no report of a synergetic effect of this combination and all tests were carried out at bottomhole temperature of 180° F. (82° C.).
Further, a cement slurry comprises in practice a whole series of additives, almost systematically among them an agent which encourages dispersion of the cement particles. The dispersing agents used vary depending on the type of wellbores or, more exactly, depending on the temperature to which the cement is subjected. Argillaceous minerals such as bentonite are also frequently used as they can reduce the density of the cement slurry, an essential point when cementing in zones where the formation pressure is low. The question of the compatibility of each new additive with current additives, over a wide range of working temperatures and pressures, is thus fundamental, it being understood that no additive is genuinely universal.
Therefore it would be suitable to provide new fluid loss control agents that could overcome some of the drawbacks of the prior art, in particular in terms of costs.